The present invention relates to a process for manufacturing thermosetting resin particles for powder coating use having improved performance.
Much interest has been focussed on the use of powder coating technology in finishing various substrates such as automobile bodies and parts, houshold electrical appliance, building materials and so on for eliminating the emission of VOC to the environment. Resin particles for powder coating use require to satisfy with several important requirements in performance. They should not aggregate into a hard cake by blocking or premature curing during storage. They must be free-flowing and hardly susceptible to the development of electrastatic charge when feeding to the applicator pneumatically. Also, they must exhibit a good thermal flowability to form a smooth film when applied onto a substrate and then baked. For some applications, the coating films thus formed require high scratch resistance, acid rain resistance and weatherability.
However, it is hardly possible to meet all or most of these requirements simultaneously because some of these requirements are contradictory to each other. It was proposed in the past to use particles having a relatively low resin molecular weight or low Tg, or finer particles having a weight average particle size less than 10 .mu.m in order to obtain coating films of high quality appearance. However, the use of low molecular weight or low Tg resin particles tends to decrease anti-blocking property, while the use of finer particles decreases powder flowability and often causes clogging of applicators and associated pneumatic feeders.
JP-A-02178360 and U.S. Pat. No. 5,034,432 disclose powder coating particles having adhered onto their surfaces finer resin particles having a particle size substantially smaller than the particle size of the substrate resin particles in an amount sufficent for preventing the substrate particles from directly contacting with each other. By selecting an appropriate material having relatively high Tg or softening point for the finer particles, it is possible to improve the anti-blocking property to some extent. Because adhesion of finer particles to larger substrate particles are performed while both components are in dry state, for example, by dry blending, it is difficult to cover individual large particles uniformly with fine particles to prevent blocking of large particles in naked areas. The dry blend process itself sometimes can increase the incidence of collision and thus aggregation of larger particles themselves before they are well covered with finer particles. Conventional powder coating compositions are produced by blending a binder resin with a crosslinker and optional additives such as pigments, kneading the blend under heat to make a molten mass, cooling and pulverizing the mass, and classifying pulverized particles. The particles thus produced comprise a large proportion of irregular particles defining frangible corners. The dry blend process can further crash irregular particles with frangible corners to increase the proportion of particles of smaller sizes less than desirable which have an adverse effect on the powder flowability of the composition. Since the above fine resin particles are not easily rest on sharp corners or edges of large substrate particles, the method taught in the cited references is not fully effective in enhancing the anti-blocking property of the conventional powder coating compositions.
A need exists, therefore, for powder coating compositions having improved storage stability and workability yet capable of forming coating films having high quality appearance.